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While approximately 80 percent of all matter in the Universe is in the form of dark matter, a substantial fraction of ordinary matter is also "missing." The total amount of matter produced in the Big Bang is a known quantity from a variety of observations and calculations, but it doesn't seem to reside in the stars, gas, and dust that form galaxies. The most widely accepted models postulate that it either surrounds galaxies in a hot cloud, or was ejected into the intergalactic medium (IGM)—the regions of space between galaxies. Previous observations of distant galaxies have backed up the second assertion.

A new set of observations performed with the orbiting Chandra X-ray Observatory may have revealed the presence of a large cloud of hot gas surrounding the Milky Way. A. Gupta, S. Mathur, Y. Krongold, F. Nicastro, and M. Galeazzi measured absorption features from the region beyond the densest parts of our galaxy and found a signal consistent with ionized oxygen atoms with temperatures around a million degrees. Based on the thickness of the gas cloud, the researchers estimated its mass to be equivalent to 10 billion Suns—roughly the same amount as all the ordinary matter in the galaxy's disk.

A variety of observations, including precision measurements of the cosmic microwave background left over from the Big Bang, have led to a fairly complete picture of the contents of the Universe. In particular, we know how much ordinary—or baryonic—matter there is. Since those atoms could not have disappeared, they must be present in the modern Universe as well, but they don't seem to be part of the disk or bulge that make up the luminous regions of galaxies. Figuring out what happened to them has become known as the "missing baryon problem."

Theoretical models predicted that these atoms may have been ejected from the central parts of galaxies into the circumgalactic medium (CGM), a region that's outside the luminous parts, but still within the galaxy's gravitational pull. Alternately, they could have been thrown into the IGM. In either case, the atoms would have relatively high temperature but low density, which makes them more difficult to find. Attempts to spot emission and absorption from atoms at temperatures around 100,000 degrees (105°C) failed to locate the missing baryons.

Spiral Galaxy Structure

Spiral galaxies like the Milky Way are comprised of three major regions. The disk is the bit containing the spiral arms, most of the brightest stars, and the spare gas and dust. The bulge has the central supermassive black hole and older stars. The halo surrounds the galaxy and contains most of the galaxy's mass in the form of dark matter. While there are many stars in the halo, they are distributed over a huge volume. The circumgalactic medium (CGM) described in the main text is part of the halo.

The present study focused on higher temperatures. At these temperatures (106 to 107°C), atoms such as oxygen or silicon can lose several electrons due to their high-energy collisions. This places the light they emit and absorb strongly into the X-ray portion of the spectrum. For this reason, the researchers used data from Chandra observations of quasars: extremely bright jets of light from supermassive black holes in distant galaxies. These objects are intense X-ray sources, so any hot, ionized gas between the quasar and Earth would create absorption features in the quasar's spectrum.

The researchers looked specifically at absorption by oxygen atoms missing six or seven electrons, known as O VII and O VIII. (The Roman numeral is always one greater than the number of missing electrons. Neutral oxygen, with all its electrons, is known as O I; if it loses a single electron, it is known as O II; and so on.) They found clear absorption features in 21 out of 29 quasars where the data were sufficiently clean.

A major part of the analysis involved showing the absorption was from atoms near the Milky Way, as opposed to more distant gas clouds. After all, these observations don't show the gas itself, but its spectral shadow, so the researchers had to reconstruct its approximate distance from Earth and the thickness of the gas cloud. Their results indicated the oxygen was quite close to the Milky Way and very spread out.

If the atoms were uniformly distributed around the galaxy, then they would make a sphere over 600 thousand light-years across, about six times the diameter of the Milky Way's disk. Since the atoms are almost certainly not in a uniform cloud, the actual size of the cloud can't be known, so the researchers weren't able to determine whether the oxygen was in the CGM or the IGM. Either way, it is consistent with theoretical predictions.

Using the density of the cloud (again based on absorption) and assuming its shape is approximately spherical, the researchers calculated its mass to be at least 12 billion times the mass of the Sun. That's comparable to the mass of all the gas and stars in the Milky Way's disk—and similar to the mass of the "missing" baryons.

To determine whether these oxygen atoms are truly part of the missing baryons, the full shape and size of the cloud will need to be determined. That would provide a better estimate of the mass. However, the existence of a hot cloud surrounding the Milky Way is a significant discovery, and could very well help resolve the case of the missing atoms—a long-standing problem in astrophysics.

I'd like to know why oxygen atoms?If this theory holds up it essentially means that half of the baryon matter in the milkyway is oxygen.

Or am I missing something and did they just concentrate on oxygen because it's easier to prove it's existence due to it's particular properties?

I'm too lazy to actually verify that this is true, but my suspicion is elements lighter than oxygen would typically be completely ionized at the temperatures they were looking for. Oxygen would be one of the most abundant elements capable of creating x-ray absorption lines at those temperatures.

So, let me make sure I have this straight. The vast majority of the universe is made up of something that we don't know what it is. Of the stuff that we do know what it is, a big chunk of it is just missing?

This is what gets on my nerves about science. It is like the different fields of science are in all these little boxes. We just assume that our scientific thinking must be in these boxes. When it comes to astrophysics, we are supposed to start by thinking everything is a result of the big bang. What is that you say? You discovered something in astrophysics that makes no sense? Well, don't change your thinking! Keep going in the direction that makes no sense, and if anyone questions you, you just tell them "well, we just haven't figured it out YET". Just assume that the human mind is capable of understanding the nature of reality. Right...

I'd like to know why oxygen atoms?If this theory holds up it essentially means that half of the baryon matter in the milkyway is oxygen.

Or am I missing something and did they just concentrate on oxygen because it's easier to prove it's existence due to it's particular properties?

I'm too lazy to actually verify that this is true, but my suspicion is elements lighter than oxygen would typically be completely ionized at the temperatures they were looking for. Oxygen would be one of the most abundant elements capable of creating x-ray absorption lines at those temperatures.

Something wasn't communicated through the article. They are stating that the oxygen is absorbing x-rays from the quasars, but it is never clarified how they determine the mass of the cloud. Is it based on oxygen being the total mass? Is it based on commonly accepted concentrations of oxygen in gas clouds?

The article does seem to indicate that they don't believe it to be all oxygen: "To determine whether these oxygen atoms are truly part of the missing baryons"

So, let me make sure I have this straight. The vast majority of the universe is made up of something that we don't know what it is. Of the stuff that we do know what it is, a big chunk of it is just missing?

This is what gets on my nerves about science. It is like the different fields of science are in all these little boxes. We just assume that our scientific thinking must be in these boxes. When it comes to astrophysics, we are supposed to start by thinking everything is a result of the big bang. What is that you say? You discovered something in astrophysics that makes no sense? Well, don't change your thinking! Keep going in the direction that makes no sense, and if anyone questions you, you just tell them "well, we just haven't figured it out YET". Just assume that the human mind is capable of understanding the nature of reality. Right...

This is what gets on my nerves about science. It is like the different fields of science are in all these little boxes. We just assume that our scientific thinking must be in these boxes. When it comes to astrophysics, we are supposed to start by thinking everything is a result of the big bang. What is that you say? You discovered something in astrophysics that makes no sense? Well, don't change your thinking! Keep going in the direction that makes no sense, and if anyone questions you, you just tell them "well, we just haven't figured it out YET". Just assume that the human mind is capable of understanding the nature of reality. Right...

Do you have a better explanation for the disparity in mass of the Universe then? Surely you jest at the idea of abandoning the Big Bang theory, but it's been tested pretty rigorously with countless studies. And for all the hubbub about alternative theories, none of them have completely replaced it because they don't match the data completely, or at least not as well as the Big Bang theory. You also have to understand that when scientists call something a "theory" and it's been around this long, it's practically true unless something contradictory happens, which is rare. It may as well be a fact, except that no one was there to actually observe it, so there's the slim possibility it could be wrong, even if that chance is unlikely.

This might be a question with an obvious answer to some, but it's an honest one.

How are the particles maintaining their heat levels in this cloud?

I know that I don't know all the way heat is generated. This cloud is not dense, so it's not the same method as planets generate heat. Maybe it's from motion caused by gravitational pull from the galaxy it surrounds? I always thought of space as empty and cold, and anything put out there would rapidly lose it's heat due to nature preferring thermodynamic equilibrium. Obviously something else is going on here beyond my understanding. (Not hard, I'm a computer guy - not a physics guy.)

So, let me make sure I have this straight. The vast majority of the universe is made up of something that we don't know what it is. Of the stuff that we do know what it is, a big chunk of it is just missing?

This is what gets on my nerves about science. It is like the different fields of science are in all these little boxes. We just assume that our scientific thinking must be in these boxes. When it comes to astrophysics, we are supposed to start by thinking everything is a result of the big bang. What is that you say? You discovered something in astrophysics that makes no sense? Well, don't change your thinking! Keep going in the direction that makes no sense, and if anyone questions you, you just tell them "well, we just haven't figured it out YET". Just assume that the human mind is capable of understanding the nature of reality. Right...

Barring any major paradigm shift the fact is that most scientific advancement is done by improving well established theories that already explain most of the facts. It thus makes sense for scientist to be conservative about new facts and try first to fit them into said theories. The standard model is IMHO the best example of such a development and its continued success exemplifies that such approach is for the most part the wiser option.

The present study focused on higher temperatures. At these temperatures (106 to 107°C), atoms such as oxygen or silicon can lose several electrons due to their high-energy collisions.

Theoretical models predicted that these atoms may have been ejected from the central parts of galaxies into the circumgalactic medium (CGM), a region that's outside the luminous parts, but still within the galaxy's gravitational pull.

I'm having trouble with the part where a gas at 1-10 million °C is gravitationally bound to the galaxy.

This might be a question with an obvious answer to some, but it's an honest one.

How are the particles maintaining their heat levels in this cloud?

They can't get rid of it quickly by transferring it through collisions with other particles. There's just not that much "stuff" for them to bump into, and of the "stuff" that is, much of it is likely to be about as hot (i.e. the other hot ions in the cloud).

This is what gets on my nerves about science. It is like the different fields of science are in all these little boxes. We just assume that our scientific thinking must be in these boxes. When it comes to astrophysics, we are supposed to start by thinking everything is a result of the big bang. What is that you say? You discovered something in astrophysics that makes no sense? Well, don't change your thinking! Keep going in the direction that makes no sense, and if anyone questions you, you just tell them "well, we just haven't figured it out YET". Just assume that the human mind is capable of understanding the nature of reality. Right...

Do you have a better explanation for the disparity in mass of the Universe then? Surely you jest at the idea of abandoning the Big Bang theory, but it's been tested pretty rigorously with countless studies. And for all the hubbub about alternative theories, none of them have completely replaced it because they don't match the data completely, or at least not as well as the Big Bang theory. You also have to understand that when scientists call something a "theory" and it's been around this long, it's practically true unless something contradictory happens, which is rare. It may as well be a fact, except that no one was there to actually observe it, so there's the slim possibility it could be wrong, even if that chance is unlikely.

No. This is something a lot of people seem to get confused about. A theory is not, and will never be, a "fact". This is like the people that go around saying well the theory of evolution is "just" a theory, when they don't even understand what this means. Their intention is to cast doubt on it, as if it were really true it would be a law and not merely a theory.

There are several parts of scientific observation and explanation. There are laws, theories, etc. Laws are things that are facts, things that can be stated as mathematical truths, for which no known counter observation exists; a law is something which summarizes all known observations. A theory is a hypothesis, something that explains some sort of observed process or phenomenon, it is the explanation for WHY we see what we see. It's a bunch of words that are meant to explain the cause of something. By definition, it can never become a law.

As an example, the law of gravitation vs. the theory of gravitation. The law of gravitation is well known, where gravity is proportional to the inverse of the square of the distance between two objects. When Newton created his law of gravitation, he had no idea where gravity came from or what caused it, but that didn't mean he couldn't still state a law about it. It was something that summarized all known observations. It didn't need an explanation of where it came from to still be a correct summary of the observations. The true cause of gravity in fact is still unknown, and there are *multiple* theories as to what exactly is going on.

In fact, there can be multiple competing theories that explain any given phenomenon, and they may all exist for a long time until something comes along to disprove one of them. Theories are *never* proved, competing theories are only *disproved*. Just because a theory has been around for a long time and hasn't been disproved does NOT in any way mean it is a fact, or can be taken as truth.

Take for example the so-called theories of everything, which try to reconcile quantum mechanics and relativity. You have M-theory, string theory, and a whole bunch of others. They've all been around for awhile, lots of different scientists support each of them, yet no reputable scientist would ever try to claim one of them as fact, merely because it's a theory that hasn't been disproved. In fact it's obvious that many scientists have to be wrong about which theory is correct, as there are many competing theories, and at most only one of them can be right.

To summarize about theories: some theories have lots of evidence for them, some have not so much evidence, some are the best explanation we have for a given phenomenon, while some exist as one of many possible explanations for a given phenomenon. Each theory must be considered on a case by case basis how much weight you should give it *based on the evidence*. Period. The end.

Now, in the case of the big bang theory, there is a LOT of evidence to support it, and it is really the only explanation we've found that explains all the data, as you mentioned. But the *evidence* for it is why it is likely the true explanation for how the universe came about, NOT because it is a theory that has been around for awhile that hasn't been disproved. Claiming any theory is true just because it's been around a long time and hasn't been disproved is flat out wrong, and doing so leads to a lot of people getting confused about what exactly theories are and what role they play in science.

Sorry to go on a bit of a rant, but it's really important to understanding science that you understand these differences and can elucidate them to others if you're trying to explain something scientific to them.

I'm having trouble with the part where a gas at 1-10 million °C is gravitationally bound to the galaxy.

At those temperatures, the typical speeds of the oxygen atoms are ~ 10-100 km/s, well below the escape velocity of the galaxy (several 100's of km/s). Even if they were above the escape velocity, though, they might not easily escape since collisions with other atoms can prevent free streaming of particles out of the galaxy. For example, most electrons and some small fraction of protons at the center of the Sun have velocities higher than the Sun's escape velocity. Those particles don't escape because they just collide with other particles long before they make it out.

What's the heat source? Friction (find that hard to believe)? Radiation (what kind, from where)? Or is that the particles have so much room their kinetic energy is easy to maintain (super low pressure?)

I always thought that in science, "law" has never meant "incontrovertible truth" but merely "relation". Many physical laws are inaccurate (Newton's law of gravity, which doesn't take relativistic effects into account) or unrealistic (Faraday's law of induction, which posits an infinitely thin wire).

"The intergalactic medium, the filamentary structure between the galaxies, is very much hotter than the average temperature of the universe: on the order of 100 thousand to 10 million Kelvin. This is because gas heats up as it falls into the intergalactic medium from the huge voids surrounding it. This temperature is enough to ionize, or strip electrons from nuclei, thus the main constituent of the intergalactic medium is ionized hydrogen. Physicists call this the Warm-Hot Intergalactic Medium. In certain areas near the intersections of cosmic filaments, the gas temperature approaches 100 million Kelvin. Although this sounds like a lot, it wouldn’t feel very hot if you were standing in it, because the atoms making it up are so sparse."

There are several parts of scientific observation and explanation. There are laws, theories, etc. Laws are things that are facts, things that can be stated as mathematical truths, for which no known counter observation exists; a law is something which summarizes all known observations

I tried to find a link to a bizarro cartoon strip, but apparently you need to pay for each use (and even to see them), so I'll just reference it.

We observe the laws of physics in this town buddy.-- Policeman standing in public square writing superman a ticket.

I think this is in the same category as

186,282 miles per second:It isn't just a good idea, it's the law!-- Unix fortune WRT the speed of light.

There is a fine distinction being missed here, the laws we have are based upon what we have seen, they are descriptive, not prescriptive. If superman existed and could fly (or goku, etc...) then our current observations would be clearly wrong (or incomplete). I've seen this kind of thing in the plots of some scifi books as well. That being said, it's interesting that up till now dark matter has been espoused as a necessity as all the normal baryonic matter had been accounted for. It now appears that this is not the case. Is there other matter that we have not seen yet? Time will tell.

Edit: apparently "It isn't just a good idea, it's the law!" has also been applied to gravity, take that superman.

A variety of observations, including precision measurements of the cosmic microwave background left over from the Big Bang, have led to a fairly complete picture of the contents of the Universe.

Yeah like this isn't going to get knocked on its ass in 50-100 years assuming we last beyond the Mayan bullshit this year and humans continue to push space exploration technology.

BkMak wrote:

cdclndc wrote:

I'm not posting anything useful until we get answers to the "Paging scientist Ringwald" reference in your last Physical Sciences article, Mr. Francis.

(The argument that could be made that I don't post anything relevant anyway is irrelevant.)

As a fellow non-relevant commenter, I believe the reference was aimed at Molly Ringwald having starred in 16 "candles".

Oh saxxy gurlfriend !!

Anyway - I'm wondering what is causing the 1 million degrees temperature in open space ? Thought it was supposed to be cold out there. Are we looking at checmical reactions or just due to the nature fo the thing is it acting more like the reverse of a star ( a star being a hot ball of material surrounded by planets -- whereas the galaxy is inside of a ball of star-like material.

Kind of like a cross between a Dyson Sphere (only natural instead of artificial) and an incubator.

Also waht is odd is that with all this technology we can somehow point out the makeup and temperature of a star 15 galaxies to the left - through all of the stellar matter between here and there and yet somehow for years and years - no one noticed a 1 million degree heat signature right here in our own backyard ? Really ?

50+ years of modern space observations - how do you not notice that ?

Many want to hold that the BIg Bang is an absolute - 13± biillion years ago - Universe is 13± billion years old - but didn't see the hot gas - wow.

How are 1 million degree temperatrues invisible ?100 degree heat in the summertime distorts light form a 1/4 mile away - wtf.

I'm sorry but the way space exploration is going these days - its starting to sound an awful lot like the King James version.

ALL of this is the equivalent of a person sitting in a crowded college football (not soccer) stadium with 90,000 other people all around them -- with both teams -- and bands and cheerleaders on the field - trying to observe another person outside of the stadium at the back of a parking lot that is full of 150,000 vehicles. Then descibing in detail the contents of the drunk bastards vomit.

You see distortion caused by rising hot air. This is pretty much how we determine temperature, by the effects of the energy on matter. It's a bit harder in space, where the medium is decidedly non-terrestrial.

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I'm sorry but the way space exploration is going these days - its starting to sound an awful lot like the King James version.

So these are oxygen atoms, with 6 or 7 electrons missing out of the usual 8 electrons. I'm no chemistry expert, but that seems to indicate two things to me:

- this is seriously reactive stuff. (an ultrastrong Lewis acid?) - this stuff has a *huge* magnetic field (8 protons and only one or two electrons)

So if you were flying through this stuff, it would fling itself towards your spaceship and then eat through your hull. Nasty stuff. Thankfully seems there's only 1 atom per (large number of cubic miles) of this stuff. Potential rocket fuel for an interstellar^H^H^H intergalactic generation ship? Bussard Ramjet fuel?

This might be a question with an obvious answer to some, but it's an honest one.

How are the particles maintaining their heat levels in this cloud?

I know that I don't know all the way heat is generated. This cloud is not dense, so it's not the same method as planets generate heat. Maybe it's from motion caused by gravitational pull from the galaxy it surrounds? I always thought of space as empty and cold, and anything put out there would rapidly lose it's heat due to nature preferring thermodynamic equilibrium. Obviously something else is going on here beyond my understanding. (Not hard, I'm a computer guy - not a physics guy.)

It is cold. A million or whatever degrees, *at these pressures*, is plenty cold to freeze water and your skin. And dry ice I would think. (But of course with no pressure it's still a bit of an academic exercise.)

So it's like an atom every 5 yards or some such. With literally nothing around them; they are zipping along at quite a pace; enough to read a million degrees when they finally impact something else. But only if you're able to analyze particles at this level in the first place. I don't think a mercury thermometer would read a million degrees if it were set adrift in this region.

That being said, it's interesting that up till now dark matter has been espoused as a necessity as all the normal baryonic matter had been accounted for. It now appears that this is not the case. Is there other matter that we have not seen yet? Time will tell.

The missing baryons is a different issue from dark matter -- even with the dark matter hypothesis to explain various observations, it has been known that some of the ordinary matter has not been accounted for. Both forms of unobserved matter are expected to be present.

Anyway - I'm wondering what is causing the 1 million degrees temperature in open space ? Thought it was supposed to be cold out there. Are we looking at checmical reactions or just due to the nature fo the thing is it acting more like the reverse of a star ( a star being a hot ball of material surrounded by planets -- whereas the galaxy is inside of a ball of star-like material.

There are several natural temperature scales, depending on how material interacts. In any case, the temperature is related to the typical speeds of particles by kT ~ mv^2. The escape velocity in the galaxy is 100's of km/s, so any material that falls into the galaxy or was present when the galaxy first formed through gravitational collapse are going to have speeds of that order. For nuclei like oxygen, that corresponds to energies of 1-10 million K. That means these temperatures are not unexpected for things that interact gravitationally. For collisionless particles that interact only gravitationally, these are the only temperatures you would expect.

For particles that interact frequently with each other through electromagnetic interactions, say dense gases in the inner galaxy, those collisions can be inelastic, with kinetic energy being lost by emitting photons, cooling the particles. For these particles, the natural temperature scale is going to be somewhere between the CMB temperature (3 K) and the temperature of any nearby bright object (the particles can absorb photons and gain energy too). For Earth, we are gaining energy from the Sun, but shedding heat (through emitting photons) off into space, so our typical temperatures are somewhere in between the 3K background temperature and the 6000K temperature of the surface of the Sun.

As for why these clouds are so hot, the density is so low and the particles are so far apart that collisions are exceedingly rare. Given a long enough time, they will eventually undergo enough collisions to cool to 3K or so, but the timescale for doing so is just a helluva lot longer than they have been around.

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Also waht is odd is that with all this technology we can somehow point out the makeup and temperature of a star 15 galaxies to the left - through all of the stellar matter between here and there and yet somehow for years and years - no one noticed a 1 million degree heat signature right here in our own backyard ? Really ?

50+ years of modern space observations - how do you not notice that ?

Many want to hold that the BIg Bang is an absolute - 13± biillion years ago - Universe is 13± billion years old - but didn't see the hot gas - wow.

How are 1 million degree temperatrues invisible ?100 degree heat in the summertime distorts light form a 1/4 mile away - wtf.

The low density has been pointed out by towermac, but I want to really stress this point. The column density of 1 meter of air is far larger than the column density of 100,000 light years of this galactic gas. That is, you look through more material in 1 meter of air than looking out through the entire galactic envelope of this gas.

Furthermore, the distortion of light above say a blacktop road on a hot day is due to the (small) change in the index of refraction for the hot air just above the road and the cooler air above it. For the ultralow density of the galactic gases, the change in index of refraction is absolutely miniscule, even if the gases are hot, so you would not see the same effect.